Short answer: On a wet day in Mevagissey: the village museum on the East Wharf (about £2, usually Easter to October), the free aquarium in the old lifeboat house (late May to October), a long lunch at the Ship Inn or the Fountain, and the Eden Project's indoor biomes about 25 minutes' drive away.
A wonderfully packed museum of village life on the East Wharf, with a Cornish kitchen, fishing history and trails for children (with a small prize). About £2 for adults, under-16s free; usually open Easter to October, 11am–4pm.
In the old lifeboat house on the quay: tanks of local sea life caught by Mevagissey's fishermen. Free, with a donation bucket; 15 minutes well spent. Late May to October.
The Ship Inn by the harbour and the Fountain Inn, the oldest pub in the village, are made for rainy afternoons. More: where to eat in Mevagissey.
About 25 minutes' drive: the biomes are warm and dry whatever the weather. Eden this winter.
Heligan's jungle valley is beautiful in soft rain, if you have waterproofs and boots.
More rainy-day ideas: St Austell on a rainy day · soft play by town · the Mevagissey guide.
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With kids this month: half term in Mevagissey.
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